With the increasing threat of global energy and serious environmental problem, the renewable energy has been developing fast. Photovoltaic industry has an extensive future as its resources are plentiful and widespread. It is an important issue to lower cost and improve efficiency for photovoltaic system.
Inverter is used to convert DC from photovoltaic arrays into AC in photovoltaic system. There are two kinds of grid-connected inverters. They are isolated and non-isolated ones. The transformer is used in the former to keep human electrically safe. But because of its power loss and huge volume, it results in many problems, such as low efficiency and power density, and high cost. Therefore, the non-isolated inverter in photovoltaic system is popular. However, there is a common mode loop circuit for the absence of transformer in the non-isolated inverter system. The high frequency and common mode current in the loop circuit leads to electromagnetic interference, and at the same time, it is dangerous to devices and human. So the high frequency and common mode current becomes a critical issue to be solved for the non-isolated inverter system.
There are two kinds of non-isolated inverters in photovoltaic system.
The first kind of inverter is symmetry in topology and it has double AC filtering inductors. Full bridge inverter circuit is usually of this kind. No extra circuit in the full bridge is needed to boost input voltage in many cases since it is enough for half of that of the half-bridge. But it is difficult for the full bridge inverter to cancel substantially the high frequency leakage current because of the parasitic factors within the inverter system. The improvements of conventional H4 full bridge circuit are made to reduce the high frequency leakage current so that it meets the industry standard. However, it costs much because the two AC filtering inductors' magnetic cores are not common for the symmetry mode.
The second kind of inverter is non-symmetry and it has single AC filtering inductor. Half bridge and mid-point clamped inverter circuits are examples of this kind. The terminal of utility grid is connected with the mid-point of DC input voltage. The parasitic capacitor voltage is constant so that the leakage current is eliminated substantially. However, it needs an extra circuit to boost input voltage as it is twice of that of the first kind. The extra circuit lowers efficiency.
So it can be seen that if the second kind of inverter can work normally without the extra boosting circuit, it has obvious advantages over the first kind in low cost and high efficiency.
For the sake of brevity, the term “full bridge inverter” as used herein is intended to refer to the said first kind of inverter and the term “half bridge inverter” is intended to represent the said second kind.
In addition, five-level inverter is becoming popular to improve efficiency.